fix: make hook tests worktree-aware by using getGitDir()

Tests were hardcoding '.git' paths directly, which fails in git worktrees
where .git is a file (not a directory) containing a pointer to the actual
git directory.

Changes:
- Replace hardcoded '.git' paths with getGitDir() calls in all hook tests
- Add os.MkdirAll() calls to ensure hooks directory exists before writing

Fixes test failures in:
- TestInstallHooks, TestInstallHooksBackup, TestInstallHooksForce,
  TestUninstallHooks, TestInstallHooksShared (hooks_test.go)
- TestDetectExistingHooks, TestInstallGitHooks_NoExistingHooks,
  TestInstallGitHooks_ExistingHookBackup (init_hooks_test.go)

Cherry-picked from PR #472

Co-Authored-By: matt wilkie <maphew@gmail.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/init_hooks_test.go

@@ -18,8 +18,11 @@ func TestDetectExistingHooks(t *testing.T) {
 		t.Skipf("Skipping test: git init failed: %v", err)
 	}
 
-	gitDir := filepath.Join(tmpDir, ".git")
-	hooksDir := filepath.Join(gitDir, "hooks")
+	gitDirPath, err := getGitDir()
+	if err != nil {
+		t.Fatalf("getGitDir() failed: %v", err)
+	}
+	hooksDir := filepath.Join(gitDirPath, "hooks")
 
 	tests := []struct {
 		name            string
@@ -109,8 +112,11 @@ func TestInstallGitHooks_NoExistingHooks(t *testing.T) {
 		t.Skipf("Skipping test: git init failed: %v", err)
 	}
 
-	gitDir := filepath.Join(tmpDir, ".git")
-	hooksDir := filepath.Join(gitDir, "hooks")
+	gitDirPath, err := getGitDir()
+	if err != nil {
+		t.Fatalf("getGitDir() failed: %v", err)
+	}
+	hooksDir := filepath.Join(gitDirPath, "hooks")
 
 	// Note: Can't fully test interactive prompt in automated tests
 	// This test verifies the logic works when no existing hooks present
@@ -148,8 +154,16 @@ func TestInstallGitHooks_ExistingHookBackup(t *testing.T) {
 		t.Skipf("Skipping test: git init failed: %v", err)
 	}
 
-	gitDir := filepath.Join(tmpDir, ".git")
-	hooksDir := filepath.Join(gitDir, "hooks")
+	gitDirPath, err := getGitDir()
+	if err != nil {
+		t.Fatalf("getGitDir() failed: %v", err)
+	}
+	hooksDir := filepath.Join(gitDirPath, "hooks")
+
+	// Ensure hooks directory exists
+	if err := os.MkdirAll(hooksDir, 0750); err != nil {
+		t.Fatalf("Failed to create hooks directory: %v", err)
+	}
 
 	// Create an existing pre-commit hook
 	preCommitPath := filepath.Join(hooksDir, "pre-commit")